The latest images from NASA's Mars Global Surveyor
spacecraft show giant plates of solidified volcanic lava and
evidence for active dunes near the planet's north pole, with
sands that have hopped or rolled across the surface in recent
months.

The images will be presented on Thursday, October 29, by
members of the mission science team at the annual meeting of the
Geological Society of America in Toronto, Canada.

The close-up views of Mars' Elysium Basin reveal the first
evidence of huge plates of solidified lava, rather than lakebed
sediments, that appear to have been broken up and transported
across the Martian surface millions of years ago as they floated
on top of molten lava. This implies that the area in the planet's
northern lowlands was once the site of giant ponds of lava flows
hundreds of kilometers across, according to Dr. Alfred S. McEwen
of the University of Arizona, Tucson, a member of the Global
Surveyor science team.

"NASA Viking mission images of the same region showed a
surface of dark plates with intervening bright surfaces that did
not quite make sense," McEwen said. "Some scientists thought they
could somehow be volcanic, while others thought they might be
related to differences in the way that wind had eroded a dried
lakebed. With these new images in hand, it is now quite easy to
understand the older, lower-resolution Viking images."

McEwen and his co-authors believe that lava erupted near
this area and the upper surface became crusted, then cooled and
cracked. Some cracks widened and portions of the surface crust
became rafts of solid rock that moved in the direction that the
molten lava was flowing underneath. Other Viking and Global
Surveyor images have shown similar plate-like lava textures in
nearby Marte Vallis, implying that some of the lava from Elysium
Basin spilled into this valley and flowed thousands of kilometers
to the northeast.

"The sparse occurrence of impact craters on these plate-like
lava surfaces suggests that the eruptions happened relatively
recently in Mars' history," McEwen explained. "These eruptions
could be much younger than the youngest of the large Martian
volcanoes like Ascraeus Mons and Olympus Mons in the Tharsis
region, but they would still have occurred many, many millions of
years ago. So these images should not be treated as evidence that
Mars is volcanically active today."

Additional close-up views of Martian sand dunes in the north
polar region are showing scientists detailed patterns of ongoing
movement of sand across the planet for the first time. Drs.
Kenneth S. Edgett, staff scientist at Malin Space Science
Systems, San Diego, CA, and Michael Malin, Mars Global Surveyor
camera principal investigator, report the presence of many fresh
dunes that have been active as recently as July or August.

"The north polar cap of Mars is surrounded by a zone of dark
dunes," Edgett said. "These were first seen by Mariner 9 as a
rippled texture, and by the Viking orbiters as definitive sand
dunes. Between late July and mid-September 1998, Mars Global
Surveyor's closest passage over the planet took us right over the
north polar dune fields four times a day. This provided us with
many opportunities to take high-resolution pictures of these
mounds."

Martian dunes typically contain granular fragments of rocks
and minerals ranging from 0.06 to 2 millimeters (0.002 to 0.08 inches)
in size, which puts them in the geologic
classification of "sand." The sand appears to have been
transported by wind in one of two ways: either by hopping over
the ground, a geological process called "saltation," or by
rolling along the ground, a process known as "traction."

Some of the dunes appear to be coated with thin, bright
frost that was left over from the northern winter season that
ended in mid-July, according to Edgett and Malin. This frost is
covered with dark streaks emanating from small dark spots that
dot the bases of many of the dunes. "The simplest explanation is
that gusts of wind have blown the dark sand out across the frost-
covered dunes, creating a streak of deposited sand over the
frost," Malin said. "Some spots seen in the close-ups have
multiple streaks, each one indicating that a different wind gust
has moved in a different direction."

Mars Global Surveyor is part of a sustained program of Mars
exploration known as the Mars Surveyor Program. The mission is
managed by the Jet Propulsion Laboratory, a division of the
California Institute of Technology, Pasadena, CA, for NASA's
Office of Space Science, Washington, DC. JPL's industrial
partner is Lockheed Martin Astronautics, Denver, CO, which
developed and operates the spacecraft.